Set TLS padding extension value.

Enable TLS padding extension using official value from:

http://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml

(Imported from upstream's 6411b83e52fdfd0d3563d50a4dc00838b142fb2c)
diff --git a/ssl/tls1.h b/ssl/tls1.h
index 79fc9c3..4ef50b7 100644
--- a/ssl/tls1.h
+++ b/ssl/tls1.h
@@ -234,6 +234,12 @@
 /* ExtensionType value from draft-ietf-tls-applayerprotoneg-00 */
 #define TLSEXT_TYPE_application_layer_protocol_negotiation 16
 
+/* ExtensionType value for TLS padding extension.
+ * http://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml
+ * http://tools.ietf.org/html/draft-agl-tls-padding-03
+ */
+#define TLSEXT_TYPE_padding	21
+
 /* ExtensionType value from RFC4507 */
 #define TLSEXT_TYPE_session_ticket		35